Recently, lithium secondary batteries have been used in various fields including portable electronic devices such as mobile phones, personal digital assistants (PDAs), and laptop computers. In particular, in line with growing concerns about environmental issues, research into lithium secondary batteries having high energy density and discharge voltage as a power source of an electric vehicle able to replace vehicles using fossil fuels such as gasoline vehicle and diesel vehicle, one of major causes of air pollution, have been actively conducted and some of the research are in a commercialization stage.
Meanwhile, in order to use a lithium secondary battery as a power source of the electric vehicle, the lithium secondary battery must maintain stable power in a wide state of charge (SOC) range along with high power.
An electric vehicle is classified as a typical electric vehicle (EV), battery electric vehicle (BEV), hybrid electric vehicle (HEV), or plug-in hybrid electric vehicle (PHEV) according to a type of the power source thereof.
However, since the typical electric vehicle (EV) and the series-type PHEV among the foregoing electric vehicles entirely depend on the battery rather than the engine in terms of driving of the vehicle, maintaining of stable power according to battery characteristics in a usable SOC range is a very important factor for driving safety in comparison to other types of electric vehicles, and a battery having a wide available SOC range is required.
Meanwhile, with respect to LiCoO2, a typical cathode material of a high-capacity lithium secondary battery, practical limits of an increase in energy density and power characteristics have been reached. In particular, when LiCoO2 is used in high energy density applications, oxygen in a structure of LiCoO2 is discharged along with structural degeneration in a high-temperature charged state due to its structural instability to generate an exothermic reaction with an electrolyte in a battery and thus, it becomes a main cause of battery explosion.
In order to improve the safety limitation of LiCoO2, uses of lithium-containing manganese oxides, such as layered crystal structure LiMnO2 and spinel crystal structure LiMn2O4, and lithium-containing nickel oxide (LiNiO2) have been considered, and a great deal of research into layered structure lithium manganese oxides, in which manganese (Mn) as an essential transition metal is added in an amount greater than those of other transition metals (excluding lithium) to layered lithium manganese oxide as a high-capacity material, has recently been conducted.
The lithium manganese oxide exhibits relatively large capacity and also exhibits relatively high power characteristics in a high SOC range. However, resistance may rapidly increase at an operating voltage limit, i.e., a low SOC range, and thus, power may rapidly decrease and initial irreversible capacity may be large.
Since there are such limitations in using typically known cathode active materials of lithium secondary batteries alone, use of a mixture formed therebetween may be required. In particular, in order to be used as a power source of medium and large sized devices, there is an urgent need for a lithium secondary battery having safety improved by exhibiting a uniform profile without a rapid voltage drop in an entire SOC range as well as having high capacity.